A. Field of the Invention
This invention relates generally to a system for scanning a three-dimensional object. The scanning system utilizes a laser-based range finding technology coupled with a scanning mirror, such as a Micro-Electro-Mechanical Systems (MEMS) mirror, oscillating at a high frequency, and a high-speed transceiver. The scanning system acquires surface information of the scanned object and generates an accurate three-dimensional computer model of the object from the captured information.
The inventive scanning system and method can be used to analyze the surface and three-dimensional shape of virtually any three-dimensional object, including work pieces and manufactured objects, art objects, archaeological artifacts, and large scale structures such as rooms and buildings. The invention is particularly useful in medical-related applications, including orthodontics, and the present document discusses the invention in the context of orthodontics and the scanning of teeth (either scanning in-vivo or a scanning a physical model). However, other uses of the scanner and method are of course within the scope of the invention.
B. Description of Related Art
Scanners are devices for capturing and recording information from a surface of an object. Scanners for obtaining information from a two-dimensional surface, such as reading bar codes or characters printed on a piece of paper, are widely known. Several scanners have been proposed for recording three-dimensional information as well.
Dentistry and orthodontics is one area where precise knowledge of a patient's dentition is desirable, and hence this is one area where three-dimensional scanners have been proposed. The key to efficiency in treatment and maximum quality in results is a realistic simulation of the treatment process. Today's orthodontists have the possibility of taking plaster models of the upper and lower jaw, cutting the cast into single tooth models and sticking these tooth models into a wax bed, lining them up in the desired position, the so-called set-up. The next step is to bond a bracket at every tooth model. This would tell the orthodontist the geometry of the wire to run through the bracket slots to receive exactly this result. To make sure that the brackets will be bonded at exactly this position at the real patient's teeth, small templates for every tooth would have to be fabricated that fit over the bracket and a relevant part of the tooth and allow for reliable placement of the bracket at the patient. To increase efficiency of the bonding process, another option would be to transfer each single bracket onto a model of the malocclusion and then fabricate one single transfer tray per jaw that covers all brackets and relevant portions of every tooth. However, it is obvious that such an approach requires an extreme amount of time and labor.
U.S. Pat. Nos. 4,837,732 and 4,575,805 to Brandestini and Moermann propose a scanning system for in vivo, non-contact scanning of teeth. The patents describe a procedure for optically mapping a prepared tooth with a non-contact scan-head. The scan-head delivers the contour data, converted to electrical format, to be stored in a memory. A computer reads the memory following a line scan pattern. A milling device is slaved to follow this pattern by means of position control signals and mills an implant for the prepared tooth cavity.
The scan-head of the '732 and '805 patents includes a light emitting diode, with integral lens that radiates light onto the cavity. Before reaching the object, the rays of light are reflected by a mirror and pass through a ruling consisting of a plurality of parallel slits, or an alternating pattern of parallel opaque and transparent stripes. A lens focuses the reflected light onto a charge-coupled device (CCD) sensor. Depth information is determined in accordance with a principle known as “active triangulation.” Basically, the object is viewed under an angle different from the incident rays due to a parallax effect. Each light stripe will have an apparent positional shift and the amount of the shift at each point along each light stripe is proportional to the vertical height of the corresponding portion of the surface on the object.
U.S. Pat. No. 5,372,502 to Massen et al. describes an optical probe for measuring teeth that works on a similar principle. As noted in the Massen et al. patent, the Brandestini et al. technique is difficult to use when there are large variations in surface topography since such large jumps displace the pattern by an amount larger than the phase constant of the pattern, making it difficult to reconstruct the pattern of lines. Furthermore, precise knowledge of the angle of incidence and angle of reflection, and the separation distance between the light source and the detector, are needed to make accurate determinations of depth. Furthermore, the scanner has to be rather carefully positioned with respect to the tooth and would be unable to make a complete model of the dentition.
U.S. Pat. No. 5,027,281 to Rekow et al. describes a scanning method using a three axis positioning head with a laser source and detector, a rotational stage and a computer controller. The computer controller positions both the rotational stage and the positioning head. An object is placed on the rotational stage and the laser beam reflects from it. The reflected laser beam is used to measure the distance between the object and the laser source. X and Y coordinates are obtained by movement of the rotational stage or the positioning head. A three-dimensional virtual model of the object is created from the laser scanning. The '281 patent describes using this scanning method for scanning a plaster model of teeth for purposes of acquiring shape of the teeth to form a dental prosthesis. The system of the '281 patent is not particularly flexible, since it requires the object to be placed on the rotational stage and precise control of the relative position of the object and the positioning head is required at all times. It is unsuited for in vivo scanning of the teeth.
U.S. Pat. No. 5,431,562 to Andreiko et al. describes a method of acquiring certain shape information of teeth from a plaster model of the teeth. The plaster model is placed on a table and a picture is taken of the teeth using a video camera positioned a known distance away from the model, looking directly down on the model. The image is displayed on an input computer and a positioning grid is placed over the image of the teeth. The operator manually inputs X and Y coordinate information of selected points on the teeth, such as the mesial and distal contact points of the teeth. An alternative embodiment is described in which a laser directs a laser beam onto a model of the teeth and the reflected beam is detected by a sensor. The patent asserts that three-dimensional information as to teeth can be acquired from this technique but does not explain how it would be done. Neither of the techniques of Andreiko has met with widespread commercial success or acceptance in orthodontics. Neither technique achieves in vivo scanning of teeth. Moreover, the video technique does not produce complete three-dimensional information as to the teeth, but rather a limited amount of two-dimensional information, requiring significant manual operator input. Even using this technique, additional equipment is required even to describe the labial surface of a tooth along a single plane.
U.S. Pat. No. 5,309,212 to Clark describes an optical scanning rangefinder that creates a depth map of its surroundings by scanning a beam of modulated, collimated light and observing reflections from proximate surfaces. The scanning system uses dual rotating prisms to deflect the transmitted beam and collect a portion of the reflected light, which is focused on a photo detector and converted to an electrical signal. This signal is amplified, AC coupled, and inverted. The inverted signal drives the modulator for the light source. When sufficient light is received by the detector, this system forms an oscillator, the frequency of which depends on the distance to the illuminated surface. This frequency is measured, and the distance to the surface derived from it.
U.S. Pat. No. 6,088,085 to Wetteborn discloses a range measurement apparatus, in particular a single beam pulsed laser range finder, comprising a light transmitter, a light receiver, an optical attenuator disposed in the transmission or reception branch and a time measuring unit for determining the light transit time between the transmission and the receipt of a light signal. The light transmitter is formed as a unitary and compact module into which a laser diode, connection elements for the laser diodes, an apparatus for coupling out the reference pulse, and also a fiber plug connector for the coupling of the transmitted light into a light conducting fiber are integrated. The light receiver is likewise formed as a unitary and compact module into which a photodiode, connection elements for the photodiode, an apparatus for coupling in the reference pulse and also a fiber plug connector for the coupling in of the received light which takes place via a light conducting fiber are integrated.
U.S. Pat. No. 6,246,468 to Dimsdale and U.S. Pat. No. 6,330,523 to Kacyra, et al. each discloses an integrated system for generating a model of a three-dimensional object. A scanning laser device scans the three-dimensional object and generates a point cloud. The points of the point cloud each indicate a location of a corresponding point on a surface of the object. A first model is generated, responsive to the point cloud, that generates a first model representing constituent geometric shapes of the object. A data file is generated, responsive to the first model, that can be inputted to a computer-aided design system.
U.S. Pat. No. 6,437,853 to Seo discloses a three-dimensional image capturing device which performs a distance measurement in first and second modes. In the first distance measurement mode, an electric charge accumulation period starts at the fall of a pulse of a distance measuring light beam, and ends after the fall of a pulse of a reflected light beam. In the second distance measurement mode, an electric charge accumulation period starts earlier than the fall of a pulse of the distance measuring light beam, by a predetermined time, and ends after the fall of a pulse of the reflected light beam. Based on a ratio of a first accumulated electric charge amount, obtained by the first distance measurement mode, to a second accumulated electric charge amount, obtained by the second distance measurement mode, the three-dimensional image is obtained.
U.S. Pat. No. 6,648,640 to Rubbert, et al. describes an interactive, computer based orthodontist treatment planning, appliance design and appliance manufacturing method and system. A scanner is described which acquires images of the dentition that are converted to three-dimensional frames of data. The data from the several frames are registered to each other to provide a complete three-dimensional virtual model of the dentition. Individual tooth objects are obtained from the virtual model. A computer-interactive software program provides for treatment planning, diagnosis and appliance design from the virtual tooth models.
U.S. Pat. No. 6,661,500 to Kindt, et al. discloses an image sensor that contains pixel cells that can individually provide a real-time output signal that is proportional to the instantaneous magnitude of incident radiation upon each pixel cell. The individual real-time output signals can be used to construct an electronic image. Additionally, the pixel cells of the image sensor can be used to collectively provide an accumulated real-time output signal that is proportional to the instantaneous magnitude of incident radiation upon a plurality of selected pixel cells. A propagated signal from a source such as a laser can be used to illuminate a target in an image. A reflection from the target can be detected in the accumulated real-time output signal. The range to the target can be determined using the round-trip propagation time between the sensor and the target of a propagated signal.
U.S. Pat. No. 6,674,895 to Rafii, et al. discloses a three-dimension distance time-of-flight system in which distance values are acquired by a plurality of sensors independently from each other. For use with this and similar systems, Z-distance accuracy and resolution are enhanced using various techniques including over-sampling acquired sensor data and forming running averages, or forming moving averages. Acquired data may be rejected if it fails to meet criteria associated with distance, luminosity, velocity, or estimated shape information reported by neighboring sensors. A sub-target having at least one pre-calibrated reflectance zone is used to improve system measurement accuracy. Elliptical error is corrected for using a disclosed method, and reversible mapping of Z-values into RGB is provided.
U.S. Pat. No. 6,697,164 to Babayoff, et al. discloses a method of determining surface topology of a portion of a three-dimensional structure. An array of incident light beams passing through a focusing optics and a probing face is shone on the portion. The beams generate illuminated spots on the structure and the intensity of the returning light rays propagating in an optical path opposite to that of the incident light rays is measured at various positions of the focal plane(s). By determining spot-specific positions yielding a maximum intensity of the returned light beams, data is generated which is representative of the topology.
Rocher, et al., “Low cost projection device with a 2-dimensioinal resonant micro scanning mirror”, MOEMS Display and Imaging Systems II, edited by Hakan. Urey, David L. Dickensheets, Proceedings of SPIE Vol. 5348 (SPIE, Bellingham, Wash., 2004) presents a demonstrator of a low cost image projection device based on a resonant 2-dimensional micro scanning mirror used for the deflection of a modulated laser beam. The mirror is operated at a low ratio of horizontal and vertical oscillation frequency. In particular, a ratio with a small shift from an integer value is used to enable a scan of the whole projection screen with a Lissajous pattern. The control circuit performs an excitation of both mirror axes by driving them with fixed frequency according to the response curves of the actuator. Programmable counters are used to generate the driving frequencies and to determine the actual beam position during the scanning process. That enables a very simple and low cost control circuit. A micro scanning mirror, fabricated at Fraunhofer IPMS, was used in the demonstrator set up. It is operated at oscillation frequencies of 1.4 kHz (slow axis) and 9.4 kHz (fast axis). The control circuit was realized and successfully tested with a FPGA implementation. The image resolution provided by the control circuit is 256×256 pixels.
Kilpelä, in a 2004 thesis entitled “Pulsed time-of-flight laser range finder techniques for fast, high precision measurement applications” from Department of Electrical and Information Engineering, University of Oulu, P.O. Box 4500, FIN-90014, Oulu, Finland, describes the development of high bandwidth (˜1 GHz) TOF (time-of-flight) laser range finder techniques for industrial measurement applications in the measurement range of zero to a few dozen meters to diffusely reflecting targets. The main goal of the thesis has been to improve single-shot precision to mm-level in order to shorten the measurement result acquisition time. A TOF laser range finder consists of a laser transmitter, one or two receivers and timing discriminators, and a time measuring unit. In order to improve single-shot precision the slew-rate of the measurement pulse should be increased, so the optical pulse of the laser transmitter should be narrower and more powerful and the bandwidth of the receiver should be higher without increasing the noise level too much.
What the art has lacked is a high-speed, reliable, accurate, low-cost, and easily used scanning system that can quickly and automatically acquire three-dimensional information of an object, without requiring substantial operator input, and in particular one that can be held in the hand and used for in vivo scanning or scanning a model. The present invention meets this need.